1. Field of the Invention
The subject invention relates generally to methods for providing alpha particle protection for integrated circuit die, and, in particular, to a method for providing alpha particle protection for an integrated circuit die prior to the plastic encapsulation thereof.
2. Description of the Prior Art
The phenomenon of charge carrier generation as a result of the passage of an alpha particle through the active surface of an integrated circuit die has been known for many years. However, the effects were generally considered to be laboratory curiosities prior to the advent of high density, dynamic MOS memories wherein the stored data is represented by the presence or absence of quantities of charge stored on capacitor portions of the storage cells. As improvements in fabrication processes allowed the shrinkage of the storage cells, the quantity of charge distinguishing a "zero" from a "one" shrank relative to the number of charge carriers generated by a typical alpha particle impact. In April of 1978, T. C. Mays and M. H. Woods with the physical analysis laboratory of Intel Corporation first reported the protection of "soft" errors induced by alpha particles in such dynamic memories. (Proceedings of the 1978 International Reliability Physics Symposium; see also "Alpha-Particle-Induced Soft Errors in Dynamic Memories," I.E.E.E. Transactions on Electron Devices, Vol. EE-26, January 1979).
Since 1978, efforts to prevent alpha particle induced errors in integrated circuits have focused on three approaches: (1) to design the circuitry, particularly charge storage cells, with sufficiently high operating margins to ensure that the quantity of charge carriers generated by even very energetic alpha particles will be insufficient to effect the logic state; (2) packaging the integrated circuit using materials having very low levels of radioactive impurities such as uranium and thorium; and (3) protecting the active surface of the integrated circuit with a material which absorbs the alpha particles before they reach the surface. A fourth alternative, not available to the integrated circuit manufacturer, is to provide an error detection and correction in the system in which the sensitive circuit is used. Since alternative 1 typically requires an increased charge storage capability, the resulting increase in die size will often be prohibitive. Alternative 2, while appearing to be the most advantageous, is very difficult to achieve and, with respect to plastic encapsulation, is presently unavailable due to the lack of any suitable filler material with sufficiently low levels of contaminants. As a result of these and other limitations, attention has been focused upon alternative 3 in an effort to identify one or more techniques for shielding the surface of the integrated circuit from the alpha particles generated within the package material itself.
In general, surface shielding has taken one of two forms: (1) a preformed tape of a suitably dense flexible material, such as polyimide, which can be adhered to the active surface of the integrated circuit prior to packaging; and (2) a liquid material, such as a liquid polyimide precursor compound, which is dispensed upon the active surface of the die and allowed to spread over the critical area and then cured prior to packaging. Development of the tape process has been severely impacted by the difficulty in physically attaching the tape to the integrated circuit die without damaging either the die, the tape or the wire bonds connecting the circuit to the adjacent portions of the lead frame, and by difficulty in achieving consistant adhesion between the tape and the die so as to preclude separation during and after packaging. With respect to the liquid coating process, problems include poor coating to die surface adhesion as a result of improper formulation or contaminated die surfaces, insufficiently thick coating after curing to intercept alpha particles, and excessively thick coatings after curing resulted in shrink-induced separation or damage to the wire bonds or to die itself. However, at least one manufacturer, Hitachi Semiconductor Division of Japan, is currently marketing 64 K memory devices having a polyimide coating applied by a proprietary process using a liquid polyimide precursor compound currently marketed by the Hitachi Chemical Company of Japan. However, the Hitachi Chemical Company polyimide compound, while generally suitable for use in side braze packages, is generally unsuitable for use in hermetic cerdip packaging due to the intolerance of the required seal temperatures and the tendency to generate intolerable levels of internal cavity moisture. In addition, Hitachi Chemical Company offers no polyimide compound suitable for use of an integrated circuit die to be plastic encapsulated.